Timing devices are used as clock sources in a variety of modern electronic circuits. Such timing devices provide frequency control and timing for applications ranging from relatively simple crystal oscillators for mobile phones and radio transmitters to more complex timing devices for computers and navigational aids.
The industry standard for portable clock applications is quartz crystal tuned oscillators (XOs). The domination of XO's in the commercial market is a result of their good relative frequency accuracy, low frequency drift (or shift) as a function of temperature, and low noise. However, while the density of electronics has grown exponentially following Moore's law, the area and volume occupied by quartz crystals has not scaled accordingly.
To address the scaling issue for XOs, there have been efforts directed toward replacing the XOs with silicon microelectromechanical (MEMS)-based resonators (or oscillators) as the frequency source for clocks. Developments in semiconductor process technology, packaging, and the integration of circuitry have enabled some progress for MEMS resonators. MEMs resonators are effectively time-base generators, or timing references, similar in operating principle to a mechanical tuning fork.
One of the most highly touted features of MEMS resonators is the integration they promise for integrated circuit (IC) timing circuits. Fabricated in silicon with bulk etching processes, MEMS resonators can in principal be tied to oscillator circuits and PLLs on the same silicon (or other semiconductor) substrate. This IC combination would allow the clock and timing generators, together with the resonator, to occupy a single low-profile semiconductor package. This package, moreover, can support high-volume assembly techniques. Even where MEMS resonators and oscillators are on separate chips, they can be in the same package, such as a stacked package, one that would be smaller and easier to handle than the metal “cans” or bulky ceramic packages that currently house crystal oscillators.
One class of frequency reference components that has the potential to meet the above-described timing device needs is based on bulk acoustic wave (BAW) devices. BAWs use the piezoelectric effect to convert electrical energy into mechanical energy resulting from an applied RF voltage. BAW devices generally operate at their mechanical resonant frequency which is defined as that frequency for which the half wavelength of sound waves propagating in the device is equal to the total piezoelectric layer thickness for a given velocity of sound for the material. BAW resonators operating in the GHz range generally have physical dimensions of tens of microns in diameter with thicknesses of a few microns.
For functionality the piezoelectric layer of the BAW device is acoustically isolated from the substrate. There are two conventional structures for acoustic isolation. The first is referred to as a Thin Film Bulk Acoustic Resonator (FBAR) device. In a FBAR device the acoustic isolation of the piezoelectric layer is achieved by removing the substrate or an appropriate sacrificial layer from beneath the electroded piezoelectric resonating component to provide an air gap cavity.
The second known device structure for providing acoustic isolation is referred to as a Solidly Mounted Resonator (SMR) device. In a SMR device the acoustic isolation is achieved by having the piezoelectric resonator on top of a highly efficient acoustic Bragg reflector that is on the substrate. The acoustic Bragg reflector includes a plurality of layers with alternating high acoustic impedance layers and low acoustic impedance layers. The thickness of each of these layers is fixed to be one quarter wavelength of the resonant frequency. A variant of the SMR device adds a second Bragg mirror on the top of the piezoelectric resonator of BAW resonator.
It is known that the first-order temperature dependence of the frequency shift for a BAW resonator can be essentially “zeroed out” by introducing a dielectric layer (e.g., SiO2) of an appropriate thickness. Such temperature compensating layers can be added to the resonator stack so that the BAW resonator has an extra dielectric layer (e.g., SiO2) between its piezoelectric layer (e.g., AlN) and its top metal (e.g., Mo) layer. If the thickness of the dielectric layer is adjusted appropriately the dielectric layer can essentially cancel out the linear temperature coefficient of frequency (TCF) of the BAW resonator.